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A Road Not Taken - 3-Valve Airhead

Many years ago in BMWRA's On The Level magazine, Robert Hellman mentioned a rumored boxer twin prototype engine with the cylinders inclined slightly upwards. I don't recall any further information, and I long since forgot about it until I stumbled upon this patent the other day, issued to BMW in 1985.

As you can see from the image, the cylinders only look like they are inclined, due to the design of the cylinders, heads, and valve covers, and the orientation of the fins on the heads and valve covers.

Not really -- given the time required for most patents to be examined and issued at the U.S. Patent and Trademark Office (years), and the fact that the U.S. patent application claims priority to a German and an International version of the same application filed about 1 1/2 years before the U.S. application, this appears to be the result of R&D done in the late '70's/early 80's, when Bings were still King.

If you look at the rest of the drawings in this patent, this also appears to be an early precursor in the line of development which lead to the oilhead released in '93. The intake air path is through the frame -- a frame bolted to the top of the engine and extending directly to the steering head. Note also the oilhead's arrangement of an auxiliary shaft under the crank driving an oil pump at the front and gears for a chain-driven overhead valve gear train at the rear of the shaft (Fig. 1).

It also provides something of a preview of the wethead's head arrangements, as the intakes are fed straight down from the top, and the exhaust exits directly underneath the head (Fig. 2).

I can't say I agree with the overbroad brush stroke condemning BMW's engineering staff at the time.

For some reason, BMW (radial) aircraft engines refrained from going to 3+ valves per cylinder long after most other engine manufacturers - but they retained their dependability. It seems that thought process carried over to other air cooled engines including those for motorcycles. US car manufacturers took the same approach, for reasons (I speculate) associated with profit margins.

The Italians, British, and Japanese motorcycles pioneered much of the >2 valve per cylinder technology. The Japanese embraced the four valves per cylinder approach early and have reaped the benefits from that forward thinking. Of course most motorcycle/car/engine manufactures have gone that way, but it seems some were dragged into that way of thinking.

back in the late forties or early fifties AJS put 3 valve cylinder heads on their 7/R race bike,if only to have one inlet and two exaust perhaps not the smartest thing but they did it

They did that to keep the exhausts cool both by being smaller and by having an air passage between. I believe the exhaust cams (yes, cams) were bevel driven off the intake to improve the air shot at the septum.

Three valves is not the novelty here, really. For BMW this is their first cam-in-head design. Decidendly different than the Airheads at that time.

‘«£Cam-in-head‘«ō suggests a design that still retains some sort of push rod arrangement, however short the latter may be. As there is one cam actuating the valves via rockers, I think this qualifies as a typical ‘«£single overhead cam‘«ō design.

I agree that the sketches give some interesting insight into the direction of BMW R&D in the late 1970‘«÷s/early 1980‘«÷s (the U.S. patent claims priority to a German patent filed in 1981). Illustrates some good ideas.

The patent itself has one very narrow independent claim, meaning that it seeks to protect very specific elements of the invention. Great if you want to defend the patent, but not necessarily broad enough to prohibit any meaningful protection from competition ‘«Ű simple to design around. Maybe the success of the Gold Wing had BMW spooked.

‘«£Cam-in-head‘«ō suggests a design that still retains some sort of push rod arrangement, however short the latter may be. As there is one cam actuating the valves via rockers, I think this qualifies as a typical ‘«£single overhead cam‘«ō design.

I was a little bit too literal - I simply meant the cam was in the head as opposed to in the engine block as for Airheads. I think it is an overhead cam. Not novel in automotive terms but novel for Airhead BMW post 1969 motorcycles.

Paul Glaves - "Big Bend", Texas U.S.A
"The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russellhttp://web.bigbend.net/~glaves/

I'd maybe have to double check a terminolgy dictionary somewhere, but I'm fairly certain "overhead cam" means the cam is higher than the valve, or in the case of a boxer farther outboard. It often but not always refers to movement only down to the valve and not movement that changes direction via a rocker arm as this engine shows. But sometimes there are the equivalent of rocker arms with cams higher than the valves, as in many BMW car inline engines.

In this case "cam in head" is indeed the better term and I would hasten to suggest nothing about this term implies a pushrod. It by definition of course suggests a direction change in motion, however, i.e. up converted to down. My old '70s Opels were termed "cam in head" engines and looked just like the drawing above save for good old GM hydraulic lifters between the cam and rocker but no actual pushrods.

The verbiage used in a brochure for the actual Oilhead reads "... heads with high-mounted cams and extremely short pushrods."

BMW finally achieved "overhead cams" with boxers first with the HP2 and then the followon "camheads" in 2010. They were a change from single "cam in head" to dual "overhead cams" as well.

I had an 85 K100. It was a bit buzzy, but otherwise a rather nice design. Certainly it had better power characteristics and lower maintenance that an 85 airhead.

The 85 K 100 had a few faults. Wiring was a problem. Buzziness was a problem. Heat was a problem. But from all I've gathered, the engine was not a problem. The classic K bike engines are as bulletproof as any BMW engine I am aware of.

Paul Glaves - "Big Bend", Texas U.S.A
"The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russellhttp://web.bigbend.net/~glaves/

I had an 85 K100. It was a bit buzzy, but otherwise a rather nice design. Certainly it had better power characteristics and lower maintenance that an 85 airhead.

So buzzy as to make it unrideable in fact.

So buzzy it would vibrate off, i.e. break the muffler bracket.

So buzzy BMW started fitting foam handgrips.

So hot BMW had numerous warranty fixes, including eliminating the seals around the fork legs to the fairing.

So hot that you really needed the knee guards glued to your tank.

So hot that in high altitudes where I live, vapor lock was a real concern. One club member here rigged a fuel cooler.

So bad, BMW brought back the boxer twin, and the "last edition" wasn't.

So bad, an '84 Airhead is worth more than an '85 K.

Remember, the popularity of the GS was a bit of a surprise to BMW, as the original R80G/S didn't sell well at all.

First one I drove off the dealer lot to test I was in top gear in a half city block trying to make the vibration go away. I've not considered a K-bike for a nanosecond since, although I know they've improved them a lot.